1. FIELD OF THE INVENTION
The present invention relates to steel powder for powder metallurgy, and particularly to sintered steel that can be manufactured therefrom, and more particularly to atomized steel powder having excellent machinability and to sintered steel that can be manufactured therefrom.
More particularly the present invention relates to atomized steel powder and produced sintered steel each having excellent machinability, satisfactory dimensional accuracy and wear resistance.
2. DESCRIPTION OF THE RELATED ART
Sintered steel is often manufactured by adding and mixing copper powder, graphite powder and other elements to steel powder and by pressing and molding the mixed powder to get desirable shaped green compact in a mold to make a sintered machine part. Such parts or the like usually have a density of about 5.0 g/cm.sup.3 to 7.2 g/cm.sup.3.
Although the powder metallurgy can produce a sintered body exhibiting excellent dimensional accuracy and having a complicated shape, parts requiring precision dimensional accuracy require a machining process, such as cutting or drilling after sintering. Accordingly, excellent machinability are often required.
In general, a product manufactured by powder metallurgy cannot easily be cut. Tools made by powder metallurgy often suffer from shorter life than those manufactured by melting processes. Machining, of course, increases the cost.
Products manufactured by powder metallurgy cannot easily be cut, at least partially, because presence of pores causes a non-continuous contact between the work piece and the cutter edge, or lowers thermal conductivity and thus raises the temperature at the site of the cutting process.
Efforts have heretofore been made to improve machinability by mixing a free cutting additive, such as S or MnS, with the steel powder. Because it acts as a lubricant reducing built-up edge formation or chip breaker.
In order to introduce S or MnS into the steel powder, Mn and S or MnS must be present in molten steel before it is atomized and formed into steel powder.
Japanese Patent Publication No. 3-25481 has suggested steel powder for powder metallurgy of a type characterized in that a small quantity, that is 0.1 wt % to 0.5 wt % of Mn, Si, C and the like are contained in molten steel, and S is added by 0.03 to 0.07 wt %, and they are sprayed with water or a gas. However, detailed performance of the steel powder has not been clarified.
Japanese Patent Publication No. 4-72905 has disclosed a free-cutting-type sinter forged part which contains two or more metal elements selected from a group consisting of 0.1 wt % to 0.9 wt % of Mn, 0.1 wt % to 1.2 wt % of Cr, 0.1 wt % to 1.0 wt % of Mo, 0.1 wt % to 2.0 wt % of Cu, 0.1 wt % to 2.0 wt % Ni, one or more elements selected from a group consisting of Nb, Al and V, S, C and Si.
Since sinter forged parts substantially establishe a true density, it is understood that substantially no pore is present. Therefore the machinability of the steel do not deteriorate due to reduction of thermal conductivity or interrupted cutting caused by pores. However, no discussion is made in the reference about ordinary sintered products of a type having a density of about 5 0 g/cm.sup.3 to 7.2 g/cm.sup.3 and including pores.
Sintered steel for powder metallurgy is ordinarily manufactured by adding and mixing Cu powder, graphite powder and the like to steel powder, by pressing the mixed powder to get a desirable shaped green compact in a mold and by sintering the green compact. The thus-manufactured sintered steel is applied for a sintered machine part or the like usually having a density of about 5.0 g/cm.sup.3 to 7.2 g/cm.sup.3 since a machine part of the foregoing type is manufactured from a long process including mixing copper powder and graphite powder with the steel powder, moving, transporting, molding and sintering, the dimensional change stability of the obtained sintered body can be deteriorated. Accordingly, a dimension controlling or correction process called a "sizing process" is usually provided after the sintering process.
However, such a sintered body is much too strong to be subjected to sizing for the purpose of correcting its dimensions. The dimensions cannot satisfactorily be corrected because of spring-back of the sintered body. The sizing process is also a costly and time-consuming additional process.
Accordingly, technologies for maintaining dimensional accuracy without sizing have been suggested. One is disclosed in Japanese Patent Publication No. 56-12304. The powder size distribution is rated to improve dimensional accuracy. In Japanese Patent Laid-Open No. 3-142342, dimensional changes due to sintering are estimated in accordance with the shape of the powder and the estimate is used to control dimensional accuracy.
On the other hand, influence of the composition of iron powder upon dimensional changes has been considered in Japanese Patent Publication No. 3-25481. A content of S is, by 0.03 wt % to 0.07 wt %, added to pure iron powder containing Mn by 0.1 wt % to 0.5 wt %, Si, C and balance iron, to prevent distortion caused by the sintering process, so as to decrease the ratio of article of dimensional interior quality taking place after sizing. The effect obtainable from the addition of S to iron powder has been mainly used to improve the machinability of the sintered body as well as preventing distortion of the sintered body disclosed in Japanese Patent Publication No. 3-25481. Improvement of machinability is also included in Japanese Patent Publication No. 3-25481.
Although disclosures have been made in Japanese Patent Publications No. 54-0457, 47-39832, 56-45964 and 61-253301, in each of which the machinability were intended to be improved by adding S to iron powder, no suggestion has been made that it can influence stability of dimensional changes.
In addition, dimensional changes take place excessively in performing sintering, in an actual manufacturing operation, because the added copper powder and graphite powder segregate easily when the powder is subjected to a movement. Movement is necessary to change a container after copper powder, graphite powder, lubricating agent and other materials have been added and mixed with steel powder. Movement is also required for various handling processes, such as transportation or supplying the mixed powder to a molding apparatus.
The degree of dimensional change undesirably varies, depending upon changes of the sintering conditions, as exemplified by sintering time and sintering temperature, for example.
However, disclosures made in, for example, Japanese Patent Publication No. 3-25481, are incapable of overcoming the problem of segregation or dimensional changes occurring in actual operation; they are due to various inevitable relevant factors.
The powder metallurgy product must usually possess good wear resistance in addition to the aforementioned characteristics. In many cases, it is conventional to add Cr. However, steel containing Cr is hardened excessively when sintered, and its machinability deteriorate. However, sintered bodies containing Cr are also required to have improved machinability.
Japanese Patent Laid-Open No. 61-253301 discloses alloy steel powder made by mixing water-sprayed mother alloy powder previously formed into an alloy with powder manufactured by roughly reducing an iron monoxide such as iron ore or mill scale, by using powder cokes serving as reducing agents; adjusting the mixture elements to desired quantities obtained after finishing reducing operation; and finish-reducing the mixed powder in a reducing atmosphere. In such a complicated manufacturing process, the cost cannot be reduced. What is worse, the disclosed basic performance, such as compressibility of the powder is unsatisfactory for practical use.